With the gradual depletion of subterranean and shallow subsea hydrocarbon reservoirs, the search for additional petroleum reserves is being extended to deeper and deeper waters on the outer continental shelves of the world. As such deeper reservoirs are discovered, increasingly complex and sophisticated production systems have been developed. It is projected that in the near future, offshore exploration and production facilities will be required for probing depths of 10,000 feet or more. Since bottom founded structures are generally limited to water depths of no more than about 1,500-2,000 feet because of the shear size of structure required, other, so-called compliant structures have been developed.
One type of compliant structure receiving considerable attention is a tension leg platform (TLP). A TLP comprises a semisubmersible-type floating platform anchored to the sea bed through vertical members or mooring lines called tension legs. The tension legs are maintained in tension at all times by insuring that the buoyancy of the TLP exceeds its operating weight under all environmental conditions. A TLP is compliantly restrained in the lateral directions allowing sway, surge and yaw while vertical plane movements such as heave, pitch and roll are stiffly restrained by the tension legs.
In the first commercially installed TLP built for the Hutton Field in the U.K. North Sea, installed in 485 feet of water, separate piled anchor templates were provided for anchoring the tensioned mooring elements extending from each of the four corner columns of the floating tension leg platform. A separate well template was also provided on the sea bed. Precise location of the four anchor templates and the well template with respect to each other was essential despite the fact that installation involved five separate operations to locate each individual template on the sea floor. In a deeper water installation, such a procedure involving five separate precision template locating operations would involve such a high cost as to be uneconomic if not totally impossible.
Riser elements extending from a subsea template to the well heads located on a deck of the floating platform present a problem during lateral offset of the platform. Because the riser elements are longer than the mooring elements, tensioning devices of relatively long stroke must be provided to avoid riser buckling during such lateral offset.
In drilling to penetrate subterranean oil deposits, particularly from a centralized offshore platform, it is often desirable to deviate a borehole outwardly away from the subsea template located directly under the platform. A fixed, bottom founded platform structure offers the availability of the use of curved conductors above the sea floor to give an initial angular offset in drilling deviated holes. However, a bottom founded template generally does not permit the building of angle from the vertical until a point subsequent to subterranean penetration.
A still further difficulty associated with the use of a TLP, particularly in deep water, is the storage of produced hydrocarbons. Deck storage of large amounts of produced hydrocarbons is impossible due to space and size limitations. Sales pipeline transfer of produced fluids is possible but may be uneconomic in a remote deep water installation. This is also true of a moored tanker storage facility in the vicinity of a TLP.
A TLP, as generally conceived, does not provide an adequately solid foundation for pulling in and attaching subsea flow lines through a J-tube for production sales or connection of satellite subsea wells.